Bainitic Microstructure Research Articles

Hot Working Tool Steel with Bainitic Microstructure—The Mechanical Properties at Elevated Temperature

Usually, tool steels are used in quenched and tempered conditions. Due to the phase transition from austenite to martensite and the volume change during the transformation, controlling distortion might be challenging. A newly developed steel focusing on a bainitic structure, even for bigger dimensions, shows a lower hardness at ambient temperature than conventional hot working tool steels but with a lower temperature dependency. Therefore, at service temperature, the mechanical properties are comparable to established grades. Heat treatment is simplified by a simple austenitization and cooling process without special requirements on the cooling rate. Due to a generally lower cooling rate and lower hardness compared to a quench and tempering process, the risk of cracking is reduced while machinability is improved. With the combination of good weldability and rather low hardness after rapid cooling, this grade can also be processed in additive manufacturing and is well suited for a hybrid process of conventional and additive manufacturing.

Parametric mathematical modeling and 3D response surface analysis for rod to plate friction welding of AISI 1020 steel/AISI 1018 steel

PurposeRotary friction welding (RFW) was used to solve the issues in fusion welding of rod to plate joints of low carbon steel (AISI 1020 steel/AISI 1018 steel) such solidification cracking, wider heat affected zone (HAZ), lower HAZ hardness, high residual stresses and distortion. The main objective of this investigation is to develop parametric mathematic models (PMMs), 3D response surface analysis to predict tensile strength (TS) and weld interface hardness (WIH) of rod to plate joints and correlate microstructure with TS and WIH of rod to plate joints.Design/methodology/approachThe three-factor x five-level central composite design (CCD) consisting fewer experiments was employed for designing experimental matrix. The tensile and microhardness tests were performed to evaluate mechanical performance of joints. The PMMs of TS and WIH of rod to plate joints were developed using polynomial regression equations incorporating the RFW parameters. The 3D response surfaces were developed using response surface methodology (RSM) to optimize RFW parameters for joining AISI 1020/AISI 1018 rod to plate.FindingsThe joints made using friction pressure/friction time (FRNP/FRNT) of 3.71 MPa/s, forging pressure/forging time (FRGP/FRGT) of 3.71 MPa/s and rotational speed (RTSP) of 19.99 rps exhibited higher TS and WIH of 452 MPa and 252 HV0.5. The PMMs accurately predicted TS and WIH of rod to plate joints at less than 1.5% error and 95% confidence. The RTSP revealed greater effect on TS and WIH of rod to plate joints followed by FRGP/FRGT and FRNP/FRNT. The superior TS and WIH of joints developed using optimized process parameters is correlated to the evolution of finer bainitic microstructure in weld interface due to the dynamic recrystallization of grains ensued by optimum frictional heating and plastic deformation.Originality/valueThe PMMs were developed for predicting TS and WIH of joints. The RFW parameters were optimized to enhance TS and WIH of joints. Low carbon steel rod to plates joints were developed using RFW for automotive applications without fusion welding defects. The microstructural features of low strength and high strength rod to plate joints were correlated to the TS and WIH of rod to plate joints.

Evaluation of wear resistance of a novel carbide-free bainitic steel

The effect of isothermal transformation temperatures on wear performances of a newly designed high-silicon carbide-free bainitic steel was studied. Wear tests were performed by a ball-on-disc tribometer using steel spheres as counterpart material. Friction coefficient was provided by the equipment, whereas wear rate was evaluated by the wear volume. Wear tracks on disc surfaces and worn cross-sections were analysed by means FEG-SEM, EDS measurements, XRD, roughness, and hardness tests. Bainitic microstructure offers better wear resistance compared to pearlitic-ferritic microstructure due to higher hardness, strain hardening capability, and strain-induced transformations. Adhesion and tribo-oxidational wear are the main wear mechanisms; the extent of oxidation increases increasing austempering temperature and it is maximum for samples treated at 350 °C and pearlitic-ferritic microstructure.

Microstructural, mechanical, and electrochemical analysis of carbon doped AISI carbon steels

The effect of carbon doping contents on the microstructure, hardness, and corrosion properties of heat-treated AISI steel grades of plain carbon steel was investigated in this study. Various microstructures including coarse ferrite-pearlite, fine ferrite-pearlite, martensite, and bainite were developed by different heat treatments i.e. annealing, normalizing, quenching, and austempering, respectively. The developed microstructures, micro-hardness, and corrosion properties were investigated by a light optical microscope, scanning electron microscope, electromechanical (Vickers Hardness tester), and electrochemical (Gamry Potentiostat) equipment, respectively. The highest corrosion rates were observed in bainitic microstructures (2.68–12.12 mpy), whereas the lowest were found in the fine ferritic-pearlitic microstructures (1.57–6.36 mpy). A direct correlation has been observed between carbon concentration and corrosion rate, i.e. carbon content resulted in an increase in corrosion rate (2.37 mpy for AISI 1020 to 9.67 mpy for AISI 1050 in annealed condition).

Fatigue crack propagation behavior of multiphase microstructure in a quenched and partitioned medium‐carbon bainitic steel

AbstractA medium‐carbon steel was treated by the bainitic isothermal transformation plus quenching and partitioning (B‐QP) process to obtain bainite/martensite/retained austenite multiphase microstructure, and its fatigue crack propagation (FCP) behavior was evaluated in contrast with BAT (bainite austempering) sample with fully bainite microstructure. Results show that B‐QP sample exhibits a lower FCP rate and higher fatigue threshold ΔKth (12.6 MPa·m1/2). Moreover, the FCP path of B‐QP sample displays a strongly tortuosity and more crack branching due to more filmy retained austenite (7.2%) and higher percentage of high angle misoriented boundaries (68%). The larger crack tortuosity and the secondary cracks as result of crack branching are primarily responsible for the lower FCP rate of B‐QP sample. In addition, the FCP rate curve of B‐QP sample shows a pronounced small plateauing at the near‐threshold zone, which can be ascribed to the mechanical twinning that occurred in the filmy retained austenite.

Influence of chemical segregation on bainitic microstructures in a carburized bearing steel

Bainite to austenite reversal is one of the grain refinement techniques employed in carburized steels. However, chemical segregation influences the homogeneity of the bainitic structure, which is seminal to exploit the advantages associated with austenite reversal. It is therefore important to understand the influence of chemical segregation on bainite formation, which is investigated in this work. Characterizations were performed on the microstructures obtained from the case and core regions of a carburized steel after 30 h of bainite treatment at 320 °C for two carbon compositions: 0.85 wt% C (zcase) and 0.16 wt% C (zcore). The microstructure of zcase is shown to contain bands with bainite in alloy-lean regions and martensite/austenite in alloy-rich regions. For zcore, although the chemical bands are not composed of different phases, the alloy-rich regions have a fraction of martensite-austenite (MA) islands that is twice the fraction in alloy-lean regions. Despite this difference, the austenite phase fractions in the chemical bands of zcore are low and almost similar, indicating that the MA islands are mostly martensite. From experimental results and thermodynamic and kinetic simulations, it is elucidated that a different rate of phase transformation in the chemical bands is the cause for the observed microstructural inhomogeneities.

The impacts of retained austenite on the microstructure and property changes of carbide-free bainite during sliding wear

With the rapid development of Chinese railway industry, there is an increased demand for better toughness, wear resistance and fatigue resistance of wheel/rail materials, and the development of carbide-free bainitic wheel/rail materials with stable and outstanding comprehensive properties is of great significance to the acceleration of railway development. This paper conducts sliding wear experiment on the currently developed FCB bainitic wheel steel (abbreviated as FCB in the following content) and AB1 bainitic rail steel (abbreviated as AB1 in the following content), and investigates the impacts of retained austenite (RA) on the microstructure and property changes of carbide-free bainite. During the process of sliding wear, the wear morphology of FCB is chiefly featured by abrasive wear; As wear rotations increase, adhesion wear and fatigue wear gradually appear. Compared with AB1, FCB with similar chemical composition has more RA, which causes it to undergo plastic deformation more easily during sliding wear. The deformation gives rise to higher strain in the surface layer and makes the grains caused by it much smaller, leading to white etching layers (WELs) with higher thickness and hardness more easily. Meanwhile, more RA means that more martensite phase with high thickness will be generated during the wear process, which increases the hardness of FCB. These two factors work together to make wear resistance of FCB better.

PBF-LB/M of Low-Alloyed Steels: Bainite-like Microstructures despite High Cooling Rates.

Laser-based powder bed fusion of metals (PBF-LB/M) is an emerging technology with enormous potential for the fabrication of highly complex products due to the layer-wise fabrication process. Low-alloyed steels have recently gained interest due to their wide potential range of applications. However, the correlation between the processing strategy and the material properties remains mostly unclear. The process-inherent high cooling rates support the assumption that a very fine martensitic microstructure is formed. Therefore, the microstructure formation was studied by means of scanning electron microscopy, hardness measurements, and an analysis of the tempering stability. It could be shown that additively manufactured Bainidur AM samples possess a bainitic microstructure despite the high process-specific cooling rates in PBF-LB/M. This bainitic microstructure is characterized by an excellent tempering stability up to temperatures as high as 600 °C. In contrast to this, additively manufactured and martensitic-hardened specimens are characterized by a higher initial hardness but a significantly reduced tempering stability. This shows the potential of manufacturing products from Bainidur AM for high-temperature applications without the necessity of a post-process heat treatment for achieving the desired bainitic microstructure.

Effects of Isothermal Temperature on Dislocation Density in Bainite Transformation of 4140 Steel

To relate the bainitic microstructures to the mechanical properties of steel, the average dislocation density needs to be determined. Using X-ray diffraction and diffraction line broadening analysis, this research quantifies the average dislocation density in the four bainite phase matrices, (upper bainite, upper and lower bainite mixture, lower bainite, lower bainite and martensite mixture), which are transformed in a wide range of isothermal temperatures. The effects of isothermal temperatures on the average dislocation density are assessed for different thermal dynamic driving forces in terms of activation energy and cooling rate. It is found that as isothermal holding temperature is increased, the dislocation density in the bainite matrix decreases from 1.55 × 1017 to 8.33 × 1015 (m−2) due to the reduction in the plastic deformation in the austenite in the transformation. At the same time, the activation energy required decreases only after passing the martensite and lower bainite mixed phase. A new method for better estimating the average dislocation density in bainitic steel is also proposed.

Effect of intercritical heat treatment process on the microstructure and mechanical properties of low alloy structural steels with residual element tin

ABSTRACT The intercritical heat treatment process is considered a novel technology to solve the mismatch of strength and toughness of low alloy steel containing residual elements. In the present study, the effects of initial microstructure and intercritical quenching temperature on microstructural morphology, mechanical properties and fracture appearance of the experimental tin-bearing steel have been investigated. The results show that, under the same intercritical quenching and tempering (IQ-T) heat treatment parameters, the initial microstructures of bainite and ferrite (B-F) and complete martensite (M) yield different ferrite morphologies, block and strip-like ferrite, respectively. It is found that after IQ-T process, the tensile strength of the steel containing 0.09% tin with B-F as initial microstructure is 1061 MPa, but the impact energy value is only 26 J; while the steel with M has achieved the combined properties of high strength and excellent toughness, and the optimal intercritical quenching temperature is 770°C.

Bainite plate thickness reduction and microstructure tailoring by double austempering of Al-rich 3Mn steel

The novel use of double austempering treatments in a multiphase steel to refine and homogenize the final microstructure and thus improve the material strength have been studied in the 3.3Mn-0.17C-1.6Al-0.23Mo-0.22Si alloy. The microstructural features developed after conventional isothermal austempering treatments at 450 °C and 400 °C were compared with those obtained after two-step heat treatments. These treatments consisted of a first isothermal holding at a temperature slightly above the initial Ms, that was interrupted at 25 and 50% of transformation, followed by a second stage treatment at a lower temperature to complete the bainitic transformation. One- and two-step treatments were performed in a high-resolution dilatometer, and the critical transformation temperatures and phase transformation kinetics were determined from the longitudinal changes recorded during these tests. It was shown that blocky-type austenite was almost completely eliminated after the two-step treatments, which in turn positively reduced the amount of fresh martensite from ∼6 to «1%. It was possible to keep the volume fraction of retained austenite above 10%, while reducing both the thickness of the bainitic plates and the film-like retained austenite by 20% and more than 40%, respectively. These microstructural characteristics made it possible to increase the hardness of the alloy by approximately 50 HV and yield strength by 180 MPa.

The effect of hydrogen on the fracture toughness of friction‐stir welded API 5L X70 pipeline steels

AbstractThe hydrogen embrittlement (HE) leads to severe steel degradation of mechanical properties. The hydrogen atoms diffuse into the steel and get positioned into reversible and irreversible trap sites. The pipe to transport oil and gas needs to be welded to construct long‐distance pipeline projects; thus, friction‐stir welding (FSW) has proven an excellent alternative to joining these pipelines. Therefore, this work assessed and analyzed the influence of hydrogen on the microstructure and fracture toughness of API 5L X70 steel welded by friction‐stir welding. The in‐service conditions were simulated by charging the specimen electrolytically in a 3.5% NaCl water solution with an intensity current of 2 mA·cm−2. According to fracture toughness tests, the base metal (BM) was more affected by hydrogen embrittlement than the friction‐stir zone (SZ), with a fracture toughness reduction of 20% after hydrogen charging. The SZ fracture toughness did not statistically show changes in hydrogen charging by the used times; however, the fracture mechanism changed from ductile to brittle‐like after 4 days of charging. The SZ depicted a better fracture toughness than BM due to the bainitic microstructure, a significant amount of irreversible hydrogen trapping.

Effect of Austempering on Mechanical Properties of Nb/V Microalloyed Bainitic Bearing Steel

In this study, a multi-element microalloyed GCr15Si1MoNbV bainitic bearing steel was designed by adding trace amounts of Nb and V. The bainite/martensite/retained austenite/undissolved carbide multiphase bearing steel with a short heat-treated cycle and remarkable comprehensive mechanical properties was prepared via low-temperature austempering, which was compared with a traditional quenched and tempered martensitic microstructure. The results showed that the comprehensive mechanical properties of the low-temperature bainitic microstructure of the Nb/V microalloyed bearing steel were better than those of a traditional tempered martensitic microstructure. In addition, as the isothermal temperature increased, the impact toughness of experimental steel was significantly improved, and the hardness decreased slightly. The tensile strength was improved with the increase in isothermal time. As the isothermal temperature increased, the bainite content of the experimental steel rose, and the stability of the retained austenite enhanced due to more carbon atoms being partitioned into it. The research work in this paper made a preliminary exploration for the application of Nb/V microalloyed bainitic bearing steel in the bearing field and provided some basic data for reference.

Microstructures and properties of wire and arc additively manufactured steel matrix composites with addition of WC by gravity-driven side powder feeding

Wire and Arc Additive Manufacturing (WAAM), which exhibits high deposition rate and low production cost, is a promising method for fabricating large-sized composite components. In this work, the WAAM process is applied to fabricate steel matrix composite components by introducing WC/Fe powder into the molten pool through gravity-driven side powder feeding. The effects of WC powder fraction on the microstructure, mechanical properties and wear resistance of deposited components have been investigated. The results reveal that WC particles are completely dissolved into the matrix for the specimen with 20 wt% WC powder, resulting in a fine bainite microstructure with W element. For specimens with WC powder fraction above 20 wt%, different structures of (Fe,W)6C phase, including discontinue and continue networks, fishbone type, and flocculent particles, are formed due to intensive reaction between WC particles and the matrix. The latter two structures are more likely to appear in specimens with high WC powder fraction. The specimen with low WC powder fraction exhibits quasi-cleavage fracture during the tensile test, while brittle fracture is observed for the specimen with high WC powder fraction. Tests prove that the mechanical properties and wear resistance have been improved by the addition of WC. The maximum strength, the maximum microhardness and the minimum wear loss are obtained for the specimens with WC powder fractions of 40 wt%, 100 wt%, and 80 wt% respectively. The improved properties of deposited composite components can be attributed to the synergistic effects of solution strengthening, precipitation strengthening of (Fe,W)6C phase, second strengthening of retained WC blocks and fine-grain strengthening.

Effect of Carbon Content on Variant Pairing in Bainitic Low Alloy Steel

Variant pairing in bainite was evaluated in four different commercial low alloy steels with medium to high carbon content. The steels investigated were austempered in the temperature range 275 °C to 450 °C to obtain a bainitic microstructure. It was found that the V1–V6 is the most frequent variant pairing at lower temperature, while it gradually decreases towards intermediate temperatures, and at the highest austempering temperatures the variant pairing with low misorientation boundaries such as V1–V4 and V1–V8 is the most frequent. The preferred variant pairing is the least pronounced after austempering of steels with higher carbon contents at intermediate temperature. Nonetheless, a continuous increase of variant pairing within the same Bain group was observed with increasing austempering temperature for all steels. Furthermore, it was observed that the deviation from the theoretical Kurdjumov–Sachs orientation relationship increases with increasing austempering temperature for all steels.

Effect of welding mode and filler metal on microstructure and mechanical properties of dissimilar joints of S900MC thermomechanical steel to St52 carbon steel welded by gas tungsten arc welding

In this research, S900MC thermomechanical steel was jointed to St52 carbon steel through a gas tungsten arc welding process in the continuous and pulsed modes. The welding was performed in two different weld designs of V-groove and close square, with and without using an ER70S filler metal, respectively. Furthermore, the microstructure, hardness, and tensile strength of the joints were investigated. A bainitic microstructure was observed in the weld metal of the samples jointed in the continuous mode. While the weld metal microstructure of the pulsed joints was changed from a ferritic microstructure to a tempered martensitic microstructure by implementing filler metal in the V-groove weld design. In all joints, the heat-affected zone was narrower at the S900M side and had a fine bainitic microstructure. Among all joints, the maximum hardness of 389 HV was achieved for the weld metal of V-groove joints welded in the pulsed mode. At a normal strain rate, there was no considerable contrast between the yield and tensile strength of all joints, however, the elongation of the continuous joints was better than that of the pulsed joints. Increasing the strain rate led to the drastic improvement of yield and tensile strength of the joint welded in the continuous mode without using filler metal.

Effect of Nb Addition and Heat Input on Heat-Affected Zone Softening in High-Strength Low-Alloy Steel.

The effect of both Nb content and heat input on the softening phenomenon of the heat-affected zone (HAZ) of low-alloy high-strength steel was studied through welding thermal simulation experiments. The microstructure evolution, density variation of geometrically necessary dislocation, microhardness distribution and the second phase precipitation behavior in HAZ was characterized and analyzed by combining the optical microscope, scanning electron microscope, high-resolution transmission electron microscope with microhardness tests. The results showed that the softening appeared in the fine-grain HAZ (FGHAZ) of the low-alloy high-strength steel with the polygonal ferrite and bainite microstructure. With an increase in Nb content, the FGHAZ softening was inhibited even with high heat input; however, the hardness shows little variation. On the one hand, the increase in the Nb content increased the volume fraction of high-strength bainite in the FGHAZ. On the other hand, the remarkable strengthening was produced by the equally distributed precipitation nanoparticles. As a result, the two factors were the main reason for the solution of the FGHAZ softening problem in the low-alloyed high-strength steel with the mixed microstructure of ferrite and bainite.

The Effect of Shot-Peening Time on Tribological Behavior of AISI5160 Steel

The aim of this study is to investigate the tribological behavior of disks made of AISI5160 steel that were austempered at 288 °C for 120 min until the bainite microstructure was completely formed. Then shot peening was applied for 0 (not shot peened), 1, 2, 3, 4, or 5 min. Before tribotests, x-ray diffraction (XRD) was employed to measure the compressive residual stress. A rotary test rig was employed to run the tribotests, and scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were employed to analyze the worn surfaces post tribotests. The compressive residual stress increased as the shot-peening time increased. Shot peening provided lower protection against wear than disk samples without shot peening. Wear debris increased during the tribotests on disk samples that had longer shot peening and led to more wear loss. This occurred because longer shot-peening time generated more cracks than shorter shot peening or no shot peening before tribotests. A limited amount of debris was found to protect the balls against wear; however, additional debris increased wear volume on the ball and led to plowing wear as well.

Microstructure of Ultrafine Acicular Bainite and Mechanical Properties of 3Cr2MnNiMo Mold Steel during Austempering

Herein, the effect of austempering treatment on martensite/bainite multiphase microstructure evolution and mechanical properties of 3Cr2MnNiMo mold steel are investigated, and the mechanical properties are compared with the conventional quenching and tempering (QT) treatment. It indicated that the morphology of martensite/bainite can be effectively tailored by changing the austempering temperature. With the increase of temperature from 280 to 360 °C, the microstructure changes from tempered martensite + bainite + blocky retained austenite (RA) to martensite + coarse bainite plate + blocky RA and irregular martensite/austenite (M/A) island. When the temperature reaches 420 °C, the martensite content reaches the maximum. The bainite morphology is surprisingly transformed from coarse bainite plate to ultrafine acicular shape owing to the reduction of the bainitic driving force and the sharp increase of carbon diffusion coefficient. Moreover, blocky RA and M/A islands disappear and are replaced by nanoscale filmy RA. The alternating distribution of variants has a refinement effect on the microstructure and the effective grain size reaches a minimum value. Compared with the conventional QT treatment, the tensile strength of austempering at 420 °C increased by 702 MPa, but the toughness and ductility levels are almost the same.

Basics of Quench Process Hardening of Powder Materials and Irons in Liquid Media

In the paper fundamentals of steel parts quench process in liquid media are considered which are used for designing new technologies such as intensive quenching that results in obtaining fine and nano – bainitic microstructure. These fundamentals include French law of transient nucleate boiling establishing during quenching in liquid media, law of transient nucleate boiling duration, when film boiling is absent, and self – regulated thermal process. Based on listed fundamentals, new versions of intensive austemering processes via cold liquid media are proposed. That allows obtaining fine bainitic microstructure in hardened materaials. Examples of performing austempering processes via cold liquid are provided by author of the paper. The proposed new method of quenching is combined with the martensite transformation which may appear in supercooled austenite in amount of less than 50%. Ideas of the paper can be used by engineers and scientists dealing with materials designing and heat treatment processes.